1. Field of the Invention
The present invention relates to the field of optics and, more particularly, to a wide-angle flat photographic lens objective. More specifically, the invention relates to a wide-angle flat photographic lens system which is built on an entirely new principle of combining a group or groups of flat microlenses with a group or groups of conventional optical lenses. The lens system of the invention may find application in photographic cameras, image projection systems, etc.
2. Background of the Invention
For better understanding the terminology used in the present description and principles of structure of optical systems in general, it would be advantageous to make some short introduction into the field of optical objectives.
An objective is an optical system or a part thereof that faces an object of observation or photographing and that creates a real image of the object turned 180° with respect to the object. Depending on the types of optical elements, objectives can be divided into lens-types, mirror-types, mirror-lens-types, and kinoform-type objectives. Most popular are lens-type objectives that are capable of acquiring various characteristics due to increase in the number of component lenses.
Photographic objectives or similar objectives of motion-picture cameras, TV cameras, night-vision instruments, and objectives used in television generally create reduced images of remote objects on a layer of a photosensitive material or on photoreceivers, e.g., TV picture tubes, matrices or linear photoreceivers, or photocathodes of optoelectronic devices. The scale of an image is proportional to a focal distance f′ of the objective, while illumination intensity is inversely proportional to a second power of a diaphragm number K, which is an f′/D ratio where D is a diaphragm of an inlet pupil of the objective. A value of 1/K is known as an aperture ratio. The limit value of the diaphragm number that allows correction of aberrations is K=0.5. A majority of existing objectives have K within the range of 3>K≦1.2. Photographic resolution capacity Nf of photo and motion-picture objectives depends on aberrations, as well as on resolution capacity Nc of the photosensitive layer of the reproducing medium and can be calculated with the use of the following approximated formula: 1/Nf=1/N0+1/Nc, wherein N0 is a visual resolution capacity of the objective. In a lens system, aberration is an error resulting from a failure of light rays from one point to converge to a single focus. A part of a space or surface the points of which are reproduced by the objective with a required quality is characterized by an angular field, i.e., a flat angle 2ω that corresponds to a solid angle that is coaxial with the optical axis and has the apex in the center of the inlet pupil of the objective.
Angular field of modern photo cameras is normally within the range of 40° to 70°, while in aerophoto cameras this angle may reach 140°.
A special group is objectives, which are also known as zoom lenses, the focal distance of which can be smoothly adjusted in a wide range by displacing separate lenses or groups of lenses along their optical axis. The number of lenses in such objectives may be as high as 30 or more. Such objectives are used, e.g., in transmission TV cameras, video cameras, and photo cameras. A ratio between the maximal and minimal focus distances may reach 40, or more. For decrease of optical losses, modern objectives are provided with anti-reflective coatings.
Normally, conventional wide-angle photographic objectives or lens systems have big dimensions, i.e., a lengthy objective, and therefore are inconvenient for use and storage. Another characteristic feature of a wide-angle photographic lens system is an increased diameter. This not only increases the overall radius and hence the dimension of the lens system but also significantly increase the weight of the objective as a whole.
There exist a large number of wide-angle photographic lens systems of different types, e.g., conventional photographic lens systems for photo cameras, image projecting lens systems, wide-field lithography systems, etc.
For example, U.S. Pat. No. 4,188,092 issued in 1980 to Kikuo Momiyama describes a retrofocus type lens for a photo camera having an angle of view at least 75° and F number 1:2.0. The lens includes a first lens group of a divergent type, a second lens group of a convergent type, and a third lens group of a convergent type. The first lens group includes in the order stated a positive meniscus lens, a negative meniscus lens, a positive meniscus lens, and a negative meniscus lens. The second lens group includes a positive lens, which is either a single lens or consists of a positive lens, and a negative lens cemented to each other and with a front convex face directed toward an object to be photographed. The third lens group includes a positive lens having a rear convex face directed toward an image of the object, a biconcave lens with its front surface radius smaller than its rear surface radius, a positive meniscus lens with a convex surface facing the image, and a positive lens. The biconcave lens and the positive meniscus lens are respectively replaceable with cemented doublet lenses. The lens system is characterized in that the first lens group includes meniscus lenses arranged in the order of positive, negative, positive and negative lenses, and particularly in that the third positive meniscus lens serves effectively to correct chromatic distortion aberration and chromatic coma aberration.
Another example, e.g., U.S. Pat. No. 6,084,719 issued in 2000 to Saburo Saguwara, et al. discloses a projection optical system that includes a first lens unit in which negative lenses included therein are larger in number than positive lenses included therein, and a second lens unit in which positive lenses included therein are larger in number than negative lenses included therein. In this projection system, design parameters are determined such that an off-axial principal ray intersects an optical axis at a point between the first lens unit and the second lens unit, and telecentricity is made on the second conjugate point side. The second lens unit includes a negative lens of meniscus form convex toward the second conjugate point side and a positive lens whose both surfaces are convex.
A common problem associated with wide-angle lens systems of the types described above as well as with other conventional wide-angle lens systems is that an increase in the aperture ratio of the lens system, widening of the field of observation, and improvement in resolution capacity of the optical system require an increase in the lens diameter. However, such an increase leads to more noticeable aberrations, and in order to solve the aberration problem, it is necessary to introduce into the system new optical elements. However, Increasing the number of lens elements to overcome the above-described drawbacks degrades the performance of the lens system due to adverse effects such as flare. All this significantly increases the manufacturing cost and the cost of the products.
Attempts have been made to solve the above problems and to improve conventional wide-angle lens systems, e.g., by increasing the amount of optical elements.
For example, U.S. Pat. No. 5,790,324 issued in 1998 to Cheon-Ho Park describes a wide-angle photographic lens system in which improvement in optical characteristics is achieved at the expense of complexity, increased weight, and increased cost. More specifically, the aforementioned lens system consists of seven lens elements, including combined lens elements.
One of the latest patents in this field, i.e., U.S. Pat. No. 6,545,824 issued in 2003 to Sensui Takayuki, discloses a significantly improved lens optical system, in which the number of lens elements is reduced to five along with a twice shorter length of the system as a whole. Nevertheless, while preserving the traditional structure, the lens optical system of U.S. Pat. No. 6,545,82 still remains large in size, heavy in weight, and complicated in structure. These problems will always remain until a wide-angle lens system is designed on traditional principles of wide-lens system architecture.
A trend that appeared at the end of 90's put forward an entirely new concept for the design of objectives that, on one hand, could satisfy all the requirements of modern optical objectives of high-quality photo cameras and, on the other hand, could satisfy the requirements of miniaturization. For example, modern digital cameras of megapixel's resolution have dimensions from a matchbox to a cigarette pack. The size of the built-in objectives makes it possible to arrange the entire objective within the boundaries of the camera's casing. It is understood that objectives of traditional design, i.e., those that use conventional three-dimensional optical lenses, cannot be reduced to the dimensions of a digital-camera objective without the loss of quality. If one reviews the situation on the present market of megapixel cameras, it can be seen that in this technique the amount of pixels that can be used for obtaining an image is rapidly growing from month to month while matching of the growing megapixel capacity of the objective with the optical components without the loss of image quality becomes more and more problematic. In addition to photo cameras, the modules with integrating optics assembly with the chip and wire assembly find application in such products as cell phones, video phones, notebooks, computers, toys, games, biometrics, etc.
Amkor Technology Co., Inc., Pennsylvania, USA, has developed an Image Sensor Camera Module, which is a complete camera solution that integrates an image sensor chip with DSP (digital processors), optics, passive components, and flexible circuit. Using an advance manufacturing solution, Amkor enabled the integration of chip and wire assembly with the optics assembly. This resulted in a low cost solution for a complete camera in a very small form factor with the length of the objectives, including wide-angle objectives, of about 8 mm. However, such results were achieved due to the use of traditional optical lenses having a very complicated shape with variable-sign curvatures on the same surface of the lens. It is understood that manufacture of such lenses requires the use of complicated non-trivial technology. With further increase in the pixel capacities of the CCD's or CMOS's the aforementioned manufacturing solution may confront some limitations from the side of optical component quality, especially if one tries to make the optics flat.
On the other hand, development of optical fiber systems, light-emitting diodes and laser diodes, systems of management, control, and conversion of light beams in optical communication systems, etc. gave impetus to developing new and efficient microoptical systems such as microlenses, microobjectives, collimators, etc. In principle of their operation and structure, the aforementioned optical elements are the same as respective traditional optical lenses, objective, collimators, etc., but are intended for working with optical beams of small diameters, e.g., from several tens of microns to several millimeters. Miniaturization of optical elements to the level of current microlenses led to very stringent requirements with regard to manufacturing accuracy and narrowed the allowable tolerances, e.g., on optical surfaces, to nanometric level. Recent success in this technology made it possible to produce microoptical lenses with very accurate aspherical surfaces.
A series of inventions made by Stephen Daniell (see, e.g., U.S. Pat. No. 6,721,101 issued in 2004) relates to the use of a microlens optical system for obtaining a 3-D image in the observer's sight. This technique is based on the principle of creation of parallax between the “left” and “right” images, which is perceived by the observer as a stereo effect.
The arrays used in the above inventions can be divided into two categories. The arrays of the first type uses air as a low-index material. Such arrays may be used, for example, in illuminated displays of electronic image detection, machine vision, and real-time 3D video capture. Arrays of the second use a fluoropolymer as a low-index material, and convey a great preponderance all incident light to the image plane.
More specifically, the system of U.S. Pat. No. 6,721,101 (as well as the systems of all other inventions of Stephen Daniell) is an assembly of two microlens array substrates, which in an overlapped state possess better optical characteristics than a single microlens array substrate. From the optical point of view, this system functions as follows: an object located at a finite distance from the observer is converted by the overlapped arrays into an infinitely located image, which is observed with the maximum possible angle of observation. This allows the observer, who is located on the symmetry axis of a display, to clearly see on this display two independent images of one object with the left and the right eyes.
In reality, the Daniell's system does not widen the angle of observation for the observer but rather creates a virtual effect of stereovision. In this system, the second and third surfaces of the array work as a separator of angles of incidence of light, i.e., starting from angle that exceeds a predetermined value, the light does not pass through the system but is reflected on the principle of total inner reflection, e.g., to the right eye, while the light incident at smaller angles passes through the system, is focused on the last flat plane of the lens system, and returns to the left eye.
Although the Daniell's system cannot be used for widening a real angle of observation and merely redistributes and divides the optical path of light that passes through the system for stereo effect, this system is a good example of a two-array assembly for optical purposes. The use of a sandwich composed of two overlapped film-like or plate-like arrays makes it possible to significantly reduce the geometrical dimensions of the lens system, especially in the optical axis direction.
The applicants made an attempt to solve the problems of the prior-art technique by developing a flat wide-angle objective having reduced longitudinal dimensions as compared to known objectives with the same characteristics. This objective, which is described in U.S. patent application Ser. No. 10/862,178 filed by the same applicants on Jun. 7, 2004, is intended for creating images with extremely wide angle of observation. The objective consists of the first sub-unit, which is located on the object side of the objective, intended for reduction of the field angle of light incidence onto the objective, and comprises an assembly of at least two microlens arrays with the same pitch between the adjacent microlenses and arranged with respect to each other so as to provide afocality, and second sub-unit that is located on the image-receiving side of the objective and comprises an assembly of conventional spherical or aspherical microlenses that create an image on an image receiver. Each pair of coaxial microlenses of the microlens arrays of the first sub-unit form an inverted microtelescope of Galileo. The outlet aperture of a single microtelescope is made so that spherical aberration can be minimized almost to 0, while field aberrations can be corrected by design parameters of the microlenses. The use of such an array of microtelescopes makes it possible to significantly reduce the overall dimensions of the first sub-unit of the lens system since the longitudinal dimension of a unit telescopic cell of the array is much smaller than the longitudinal dimension of a conventional lens component used for the same function.
Although the aforementioned objective of U.S. patent application Ser. No. 10/862,178 significantly reduces the overall dimensions of the first sub-unit of the objective by replacing it with a set of thin-film microlenses, the second sub-unit that consists of conventional optical lenses still comprises a set of four conventional lenses. In principle, the objective described in U.S. patent application Ser. No. 10/862,178 can be realized with the use of only two or three conventional lenses but this could be done at the expense of the image quality that will be impaired because of aberration that could not be completely eliminated. These four lenses form a main factor that determines the overall longitudinal dimension of the objective, which still remains significant. Therefore, there is still enough room for improvement in this direction.